As used herein, the term tachycardia refers to any fast abnormal rhythm of the heart which may be amenable to treatment by electrical discharges and specifically includes supraventricular tachycardia (SVT), ventricular tachycardia (VT), and ventricular flutter and ventricular fibrillation (VF).
The term cardioversion refers to the discharge of electrical energy into cardiac tissue in an attempt to terminate or revert a tachycardia. It may range from a high (40 Joules or more) to a low (less than 1 Joule) energy discharge. The discharge may be monophasic or biphasic but is not restricted to these waveforms. Cardioversion shocks may or may not be synchronized to the rhythm of the heart. Defibrillation is a particular example of cardioversion.
This invention applies equally to devices which deliver energy synchronized to an R-wave and to those that do not, and it applies to devices which use lower energy pulses (up to 1 Joule) as well as to devices which use greater energy pulses (up to 40 Joules or more). The invention applies to devices which deliver cardioverting shocks alone as well as in combination with antitachycardia pacing pulses.
U.S. Pat. No. 3,857,398 to Rubin describes a combined pacer/defibrillator. This device either performs a bradycardia pacing or a defibrillation function depending on the condition detected. If a VT/VF is detected, the device is switched to the defibrillating mode. After a period of time to charge a capacitor, a defibrillation shock is delivered to the patient.
A multiprogrammable, telemetric, implantable defibrillator is disclosed in the co-pending patent application of N. L. Gilli et , Ser. No. 239,624, filed Sep. 1, 1988 now abandoned, and entitled "Reconfirmation Prior to Shock in Implantable Defibrillator". The Gilli et al. device contains a bradycardia support system as well as a high energy shock system to revert ventricular tachycardias to normal sinus rhythm. On reconfirmation of the presence of a tachycardia, a shock is delivered to the patient, either at a predetermined time or when the desired energy level is reached.
A further development in the field of combined implantable devices is described in U.S. Pat. No. 4,940,054 to Grevis and Gilli, which issued on Jul. 10, 1990, and is entitled "Apparatus and Method for Controlling Multiple Sensitivities in Arrhythmia Control Systems Including Post Therapy Pacing Delay". This device is a microcomputer-based arrhythmia control system which is programmable by means of a telemetric link. The device provides single chamber bradycardia support pacing, antitachycardia pacing, and cardioversion or defibrillation shocks for restoring normal sinus rhythm to a patient.
Various developments have been made in tachycardia control pacers. Tachycardia is a condition in which the heart beats very rapidly, typically above 150 beats per minute (hereinafter "bpm"). There are several different pacing modalities which have been suggested for the termination of tachycardia. The underlying principle in all of them is that if a pacer stimulates the heart at least once shortly after a heartbeat, before the next naturally occurring heartbeat at the rapid rate, the heart may successfully revert to normal sinus rhythm. Tachycardia is often the result of electrical feedback within the heart. A natural beat results in the feedback of an electrical stimulus which prematurely triggers another beat. By interposing a stimulated heartbeat, the stability of the feedback loop is disrupted.
In U.S. Pat. No. 3,942,534 to K. R. Allen et al., which issued on Mar. 9, 1976, and is entitled "Device For Terminating Tachycardia," there is disclosed a pacer which, following detection of a tachycardia, generates an atrial (or ventricular) stimulus after a delay interval. If that stimulus is not successful in terminating the condition, then another stimulus is generated after another premature heartbeat following a slightly different delay. The device constantly adjusts the delay interval by scanning through a predetermined delay range. Stimulation ceases as soon as the heart is restored to sinus rhythm. If successful reversion is not achieved during one complete scan, then the cycle is repeated. The device further provides a second stimulus following the first, both stimuli occurring within the same tachycardia cycle, i.e. before the next naturally occurring rapid beat. The time period between a heartbeat and the first stimulus is known as the initial delay, while the time period between the first stimulus and the second stimulus is known as the coupled interval. In the above device, once the coupled interval is set by a physician it is fixed, and therefore the second stimulus always occurs a predetermined time after the first stimulus, no matter when the first stimulus occurs after the last heartbeat.
U.S. Pat. No. 4,390,021 to R. A. J. Spurrell et al., which issued on Jun. 28, 1983, and is entitled "Two Pulse Tachycardia Control Pacer," discloses a pacer for controlling tachycardia in which the coupled interval is scanned in addition to the initial delay. The time parameters which are successful in terminating the tachycardia is stored so that upon confirmation of another tachycardia event, the previously successful time parameters are the first ones to be tried. The device also allows tachycardia to be induced by the physician to allow for programming of the initial delay and the coupled interval parameters.
U.S. Pat. No. 4,398,536 to T. A. Nappholz et al., which issued on Aug. 16, 1983, discloses a scanning burst tachycardia control pacer. Following each tachycardia confirmation, a burst of a programmed number of stimulating atrial (or ventricular) pulses is generated. The rates of the bursts increase from cycle to cycle whereby following each tachycardia confirmation, a pulse burst at a different rate is generated. The rate of a burst which is successful in terminating tachycardia is stored, and following the next tachycardia confirmation, the stored rate is used for the first burst which is generated.
U.S. Pat. No. 4,406,287 to T. A. Nappholz et al., which issued on Sep. 27, 1983, discloses a variable length scanning burst tachycardia control pacer. The physician programs the maximum number of pulses in a burst. The number of pulses in a burst is scanned, and the number which is successful in terminating tachycardia is registered so that it is available for first use when a new tachycardia episode is confirmed. Successive bursts, all at the same rate, have different numbers of pulses, the pulse number scanning being in the upward direction. If all bursts are unsuccessful, a new rate is tried and the number scanning begins over again. Thus all combinations of rates and pulse numbers are tried, with the successful combination being used first following the next tachycardia confirmation.
U.S. Pat. No. 4,408,606 to R. A. J. Spurrell et al., which issued on Oct. 11, 1983 discloses a rate related tachycardia control pacer. Following tachycardia confirmation, a burst of at least three stimulating pulses is generated. The time intervals between successive pulses decrease by a fixed decrement; hence the rate of the pulses increases during each cycle of operation. The first pulse is generated following the last heartbeat which is used to confirm tachycardia, at a time which is dependent on the tachycardia rate. The time delay between the last heartbeat and the first pulse in the burst is equal to the time interval between the last two heartbeats less the fixed decrement which characterizes successive time intervals between stimulating pulses.
The problem with the storage of successful antitachycardia pacing parameters as described by Spurrell et al. U.S. Pat. No. 4,390,021, Nappholz et al. U.S. Pat. No. 4,398,536, Nappholz et al. U.S. Pat. No. 4,406,287 and Spurrell al. U.S. Pat. No. 4,408,606 is that a subsequent episode of tachyarrhythmia may be triggered in a different circuit from that in which a previous episode was pace-terminated. Accordingly, the recalled parameters will not necessarily be appropriate for the new tachyarrhythmia.
Depending on the relative differences in circuit path length as well as the direction of scanning, it may be that starting from inappropriate recalled parameters will cause even more pacing trains to be taken to reach reversion than would be the case if scanning had been started from zero.
Commonly, a patient will have more than one triggerable reentrant circuit. It is an object of the present invention to discriminate between different reentrant circuits and to associate different stored sets of successful antitachycardia pacing parameters with different circuits.